Isocyanates, especially diisocyanates, are prepared predominantly by phosgenating the corresponding amines. The phosgenation can be performed either in the liquid phase or in the gas phase. In industrial implementation, gas phase phosgenation has a series of advantages over liquid phase phosgenation, especially a higher selectivity, a lower holdup of toxic phosgene, and lower capital and energy costs.
The phosgenation of amines to isocyanates in the gas phase is known per se, for example from EP-A 570 799, EP-A 749 958 and EP-A 1 078 918. This involves evaporating one amine-containing and one phosgene-containing reactant stream, if they are not already in the gas phase, and bringing them to the reaction temperature of the gas phase phosgenation, for example of about 300 to 400° C. The provision of the high-temperature heat according to the known processes is very costly.
The gaseous product stream has to be cooled after the reaction. It is known from the prior art that the reaction gases have to be cooled rapidly in order to very substantially avoid the formation of undesired conversion products. In the prior art, what is known as quenching is predominantly used for that purpose. Quenching involves cooling by means of direct contact with a cooling liquid, which is preferably sprayed into the stream of the hot reaction gases, as described, for example, in EP-A 1 403 248. This ensures rapid cooling, i.e. a short cooling time, such that the undesired formation of conversion products can be substantially prevented.
EP-A 1 935 875 discloses, for example, a process for preparing isocyanates in the gas phase, in which the reaction is stopped by conducting the reaction mixture out of the reaction chamber through a cooling zone into which liquids are sprayed, such that the reaction gases are cooled directly. According to this published specification, indirect cooling by means of heat exchangers is disadvantageous owing to the poor heat transfer and also leads to the formation of deposits of solids on the comparatively cold surfaces of the heat exchangers as a result of side reactions of the gas mixture on these surfaces. To avoid these disadvantages, EP-A 1 935 875 proposes direct cooling by means of quenching.
However, the cooling processes known from the prior art, owing to the rapid cooling of the reaction gases and the high temperature differences from the heat-absorbing medium, especially in the course of quenching, often result in undesired aerosol formation. Aerosols formed can be removed again only with difficulty by conventional industrial processes and increase the complexity of the further workup to purify the individual components.
Furthermore, the waste heat which is absorbed by heating or evaporation of the quench liquid in the direct cooling processes of the prior art can be utilized only at a relatively low temperature level. When indirect heat transferers are utilized, further utilization of the waste heat is possible in principle. In this case, it would be desirable to heat the heat carrier medium, which absorbs the heat, to such an extent that a high temperature level is attained, which ideally corresponds completely or approximately to the temperature of the reactant streams used in the reaction zone or to the evaporation temperature of one or more reactants. This could allow the heat to be stored at a high temperature level. This enables universal reuse of the waste heat in the process.